Tetragonal tungsten bronze oxides Pb2K1-xLixNb5O15 (x=0.7) was found to exhibit a tunable novel negative thermal expansion (NTE) property, which may correspond to the both ferroelectric volumetric effect and semi-open flexible framework structure. The critical role of lithium in modulating the thermal expansion of Pb2KNb5O15 is unclear because of the difficulty in determining the accurate positions of lithium. To elucidate the the mechanism of NTE property and accordingly to sort out the position of lithium, an series of high quality neutron powder diffraction pattern at various temperature is desired.